The present invention concerns real time and off line signal processing of a signal indicating plasma concentration within a processing chamber.
In a processing chamber, for example an etch chamber used for performing a reactive ion etch in the presence of a rotating magnetic field, the concentration of plasma within the processing chamber is monitored to determine the end point of the process. Light intensity within the processing chamber is monitored using a photo multiplier tube (PMT). The light intensity from the glow of the plasma is proportional to concentration of plasma within the processing chamber. The PMT generates an electrical signal with a voltage which is proportional to the light intensity. This signal is amplified and forwarded to a computer. The computer uses the signal to determine the end point of the process, i.e., when processing is to be stopped.
When a rotating magnetic field is used in the process, this causes plasma in the processing chamber to circulate. The circulation of plasma causes significant variations in the light intensity measured by the PMT. In order to accurately determine the end point of the process, the computer needs to filter the signal from the PMT to remove the noise caused by effects of the circulation. Further it is desirable to generate a display to a user which represents the method by which the computer determines the end point of the process.